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374 lines
14 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Chapter 4. Concurrency</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB, Java Edition Transaction Processing" />
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<link rel="up" href="index.html" title="Getting Started with Berkeley DB, Java Edition Transaction Processing" />
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<link rel="prev" href="maxtxns.html" title="Configuring the Transaction Subsystem" />
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<link rel="next" href="blocking_deadlocks.html" title="Locks, Blocks, and Deadlocks" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.2.7.5</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Chapter 4. Concurrency</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="maxtxns.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="blocking_deadlocks.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="chapter" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title"><a id="txnconcurrency"></a>Chapter 4. Concurrency</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<p>
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<b>Table of Contents</b>
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</p>
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<dl>
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<dt>
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<span class="sect1">
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<a href="txnconcurrency.html#concurrenthandles">Which JE Handles are Free-Threaded</a>
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</span>
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</dt>
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<dt>
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<span class="sect1">
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<a href="blocking_deadlocks.html">Locks, Blocks, and Deadlocks</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="blocking_deadlocks.html#locks">Locks</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="blocking_deadlocks.html#blocks">Blocks</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="blocking_deadlocks.html#deadlocks">Deadlocks</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="jelock.html">JE Lock Management</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="jelock.html#jelocktimeout">Managing JE Lock Timeouts</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="jelock.html#jedeadlock">Managing Deadlocks and other Lock Conflicts</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="isolation.html">Isolation</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="isolation.html#degreesofisolation">Supported Degrees of Isolation</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="isolation.html#dirtyreads">Reading Uncommitted Data</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="isolation.html#readcommitted">Committed Reads</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="isolation.html#serializable">Configuring Serializable Isolation</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="txn_ccursor.html">Transactional Cursors and Concurrent Applications</a>
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</span>
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</dt>
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<dd>
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<dl>
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<dt>
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<span class="sect2">
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<a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</a>
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</span>
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</dt>
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</dl>
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</dd>
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<dt>
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<span class="sect1">
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<a href="readmodifywrite.html">Read/Modify/Write</a>
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</span>
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</dt>
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</dl>
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</div>
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<p>
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JE offers a great deal of support for multi-threaded
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applications even when transactions are not in use. Many of JE's
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handles are
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thread-safe
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and JE provides a
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flexible locking subsystem for managing databases in a concurrent
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application. Further, JE provides a robust mechanism for
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detecting and responding to
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<span>lock conflicts</span>.
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All of these concepts are
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explored in this chapter.
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</p>
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<p>
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Before continuing, it is useful to define a few terms that will appear
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throughout this chapter:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<span class="emphasis"><em>Thread of control</em></span>
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</p>
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<p>
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Refers to a thread that is performing work in your application.
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Typically, in this book that thread will be performing JE
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operations.
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</p>
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</li>
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<li>
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<p>
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<span class="emphasis"><em>Locking</em></span>
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</p>
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<p>
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When a thread of control obtains
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access to a shared resource, it is said to be
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<span class="emphasis"><em>locking</em></span> that resource. Note that
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JE supports both exclusive and non-exclusive locks. See
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<a class="xref" href="blocking_deadlocks.html#locks" title="Locks">Locks</a> for more information.
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</p>
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</li>
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<li>
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<p>
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<span class="emphasis"><em>Free-threaded</em></span>
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</p>
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<p>
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Data structures and objects are free-threaded if they can be
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shared across threads of control without any explicit locking on
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the part of the application. Some books, libraries, and
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programming languages may use the term
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<span class="emphasis"><em>thread-safe</em></span> for data structures or objects
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that have this characteristic. The two terms mean the
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same thing.
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</p>
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<p>
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For a description of free-threaded JE objects, see
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<a class="xref" href="txnconcurrency.html#concurrenthandles" title="Which JE Handles are Free-Threaded">Which JE Handles are Free-Threaded</a>.
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</p>
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</li>
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<li>
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<p>
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<span class="emphasis"><em>Blocked</em></span>
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</p>
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<p>
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When a thread cannot obtain a lock because some other
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thread already holds a lock on that object, the lock
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attempt is said to be <span class="emphasis"><em>blocked</em></span>. See
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<a class="xref" href="blocking_deadlocks.html#blocks" title="Blocks">Blocks</a> for more information.
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</p>
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</li>
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<li>
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<p>
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<span class="emphasis"><em>Deadlock</em></span>
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</p>
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<p>
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Occurs when two or more threads of control attempt to access conflicting resource in such a way as none
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of the threads can any longer make further progress.
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</p>
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<p>
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For example, if Thread A is blocked waiting for a resource held by Thread
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B, while at the same time Thread B is blocked waiting for a
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resource held by Thread A, then neither thread can make any
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forward progress. In this situation, Thread A and Thread B
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are said to be <span class="emphasis"><em>deadlocked.</em></span>
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</p>
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<p>
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For more information, see <a class="xref" href="blocking_deadlocks.html#deadlocks" title="Deadlocks">Deadlocks</a>.
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</p>
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</li>
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<li>
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<p>
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Lock Conflict
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</p>
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<p>
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In JE, a lock conflict simply means that a thread of
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control attempted to obtain a lock, but was unable to get
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it before the lock timeout period expired. This may have
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happened because a deadlock has occurred, or it might have
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happened because another thread is taking too long to
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complete a long-running database operation. Either way,
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you do the same things in response to a lock conflict
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as you do for a true deadlock. See <a class="xref" href="jelock.html" title="JE Lock Management">JE Lock Management</a> for more information.
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</p>
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</li>
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</ul>
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="concurrenthandles"></a>Which JE Handles are Free-Threaded</h2>
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</div>
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</div>
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</div>
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<p>
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The following describes to what extent and under what conditions
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individual handles are free-threaded.
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<code class="classname">Environment</code>
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<span>
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and the DPL
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<code class="classname">EntityStore</code>
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</span>
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</p>
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<p>
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This class is free-threaded.
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</p>
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</li>
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<li>
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<p>
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<code class="classname">Database</code>
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<span>
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and the DPL
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<code class="classname">PrimaryIndex</code>
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</span>
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</p>
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<p>
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These classes are free-threaded.
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</p>
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</li>
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<li>
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<p>
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<code class="classname">SecondaryDatabase</code>
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and DPL <code class="classname">SecondaryIndex</code>
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</p>
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<p>
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These classes are free-threaded.
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</p>
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</li>
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<li>
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<p>
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<code class="classname">Cursor</code>
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<span>
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and the DPL
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<code class="classname">EntityCursor</code>
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</span>
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</p>
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<p>
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If the cursor is a transactional cursor, it can be used by
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multiple threads of control so long as the application
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serializes access to the handle.
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If the cursor is not a transactional cursor, it can not be
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shared across multiple threads of control at all.
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</p>
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</li>
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<li>
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<p>
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<code class="classname">SecondaryCursor</code>
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</p>
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<p>
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Same conditions apply as for <code class="classname">Cursor</code>
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handles.
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</p>
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</li>
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<li>
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<p>
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<code class="classname">Transaction</code>
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</p>
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<p>
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This class is free-threaded.
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</p>
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</li>
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</ul>
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</div>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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<p>
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All other classes found in the DPL
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(<code class="literal">com.sleepycat.persist.*</code>) and not
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mentioned above are free-threaded.
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</p>
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<p>
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All classes found in the bind APIs (<code class="literal">com.sleepycat.bind.*</code>) are free-threaded.
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</p>
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</div>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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||
<table width="100%" summary="Navigation footer">
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||
<tr>
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||
<td width="40%" align="left"><a accesskey="p" href="maxtxns.html">Prev</a> </td>
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||
<td width="20%" align="center"> </td>
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||
<td width="40%" align="right"> <a accesskey="n" href="blocking_deadlocks.html">Next</a></td>
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||
</tr>
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<tr>
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<td width="40%" align="left" valign="top">Configuring the Transaction Subsystem </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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||
</td>
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<td width="40%" align="right" valign="top"> Locks, Blocks, and Deadlocks</td>
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||
</tr>
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||
</table>
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||
</div>
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</body>
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</html>
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